<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        #div1{
            width: 200px;
            height: 200px;
            background: skyblue;
            border-radius: 50%;
            position: absolute;
            right: 0;
            bottom: 0;
            top:50%;
            margin-top: -100px;
        }
    </style>
</head>
<body style="height: 2000px;">
  <div id="div1"></div>
  <script>
      var timer = null;
      var oDiv = document.getElementById('div1');
      window.onscroll = function () {
          var scroTop = document.documentElement.scrollTop || document.body.scrollTop;

          startMove((document.documentElement.clientHeight-oDiv.offsetHeight)/2+scroTop+400);

      };
      function  startMove(iTarget) {
          clearInterval(timer);
          timer = setInterval(function () {
              var speed = (iTarget-oDiv.offsetTop)/4;
              speed = speed>0?Math.ceil(speed):Math.floor(speed);

              if (iTarget == oDiv.offsetTop){
                  clearInterval(timer);
              }
              else {
                  oDiv.style.top = oDiv.offsetTop+speed+'px';
              }

          },30)

      }

  </script>
</body>
</html>